I remember people on TSR saying previously that Orange signal on campus was poor. I had just gotten myself a phone contract (yups, on Orange) and cursed quite loudly. But in Rootes it was fine. On main campus it's fine. When I changed accommodation to Tocil, it was fine.
However, in my ex's room in Tocil, Orange signal was shocking. I always thought he was making excuses but when I used my own phone there, it didn't blooming work. So like TheTallOne sort of points out with the O2 thing, it's quite a random thing (he only lived a block away from me), although in this case this consistently affected both of our phones.
Orange users definitely don't get signal in the damn Copper Rooms though! Bane of my life, trying to send a text in there.
I would strongly suggest just going with the best deal if you're looking to get a new phone, because the majority of networks work well ENOUGH throughout campus, but hardly work at all in the Copper Rooms. So even if you are lucky enough to have a phone that does work in the Copper Rooms, many of the people that you are contacting, won't.